A printer of the electrophotographic type, for example, is configured to apply light to a charged photosensitive drum in a selective manner depending on printing data to form an electrostatic latent image. Toner is deposited on the electrostatic latent image to form a toner image and the toner image is transferred to a sheet of paper, where it is fused into place.
FIG. 30 is a block diagram of a printer control circuit for use in such a conventional electrophotographic printer. FIGS. 31 and 32 are timing diagrams explaining the printing operation of the conventional electrophotographic printer. In FIG. 30, a reference numeral 1 denotes a print control unit comprised of a microprocessor, a ROM, a RAM, an input/output port, a timer and so on. The print control unit 1, which is included in a printing unit of the printer, performs sequential control over the whole of the printer in accordance with a control signal SG1 received from a not illustrated higher controller, a video signal SG2 (dot map data arranged in one dimension), etc.
The print control unit 1, upon receiving a command to start printing included in the control signal SG1, determines whether the fixing unit 77 with a built-in heater 77a is within an operable temperature range by use of the temperature sensor 75. If the fixing unit 77 is not within the operable temperature range, the print control unit 1 passes a current through the heater 77a to heat the fixing unit 77 to this range. Then, the print control unit 1 causes the developing/transferring process motor 68 to rotate by use of the driver 67, and at the same time, turns on the charging-high-voltage source 63 to apply the voltage to the charger 64 in response to a charge signal SGC.
The paper remaining sensor 73 and the paper size sensor 74 detect the presence or absence and the size of sheets of paper set in place (not shown). The paper feeding motor 70, which is rotatable in both directions, rotates in the reverse direction first to feed the paper until it is sensed by the paper inlet sensor 71. Then, it rotates in the normal direction to let the paper in a printing mechanism of the printer.
The print control unit 1 sends a timing signal SG3 which includes a main scanning/synchronizing signal and a sub-scanning/synchronizing signal to the higher controller when the paper reaches a particular position, and then receives a video signal SG2 from the higher controller. The video signal SG2 edited page by page in the higher controller and received by the print control unit 1 is transferred to the LED head 78 as printing data HD-DATA. The LED head 78 is comprised of a plurality of LEDs arranged in a line, each LED printing one dot (one pixel).
The print control unit 1, upon receiving one line of video signals, sends a latch signal HD-LOAD to the LED head 78 to have this LED 78 hold the printing data HD-DATA. Thus, the print control unit 1 can carry out printing operation in accordance with the printing data HD-DATA held in the LED head 78 even during reception of the next video signal SG2 output from the higher controller. HD-CLK denotes a clock for sending the printing data HD-DATA to the LED head 78. The sending and receiving of the video signal SG2 is carried out for each line.
The information to be printed by the LED head 78 is formed on the not illustrated photosensitive drum that has been negatively charged as a latent image comprised of dots that are at a raised potential (approximately 0 V). The toner that has been negatively charged is absorbed into each of the dots in a developing unit, and as the result, a toner image is formed. The toner image is sent to the transfer unit 66. At this time, the transfer-voltage source 65 is turned on by a transfer signal SG4, so that the transfer unit 66 transfers the toner image to the paper passing through a gap between the photosensitive drum and the transfer unit 66.
The paper bearing the toner image is conveyed keeping in contact with the fixing unit 77 including the heater 77a, so that the toner image is fused into the paper. The paper bearing this fused image is further conveyed to pass through the printing mechanism and the paper outlet sensor 72, and discharged from the printer. The print control unit 1 applies the voltage generated by the transfer-voltage source 65 to the transfer unit 66 only while the paper is passing through the transfer unit 66 in response to signals output from the paper-size sensor 74 and the paper inlet sensor 71. When the printing operation is completed and the paper passes the paper outlet sensor 72, the application of the voltage generated by the charging-voltage source 63 to the charger 64 is ceased, and at the same time, the developing/transferring process motor 68 is stopped. The above operation is repeated thereafter.
Next, the LED head 78 will be explained. FIG. 33 shows a circuitry of the LED head. As shown in this figure, the printing data HD-DATA is input into the LED head 78 together with the clock HD-CLK. For example, if the printer supports A4 paper and has resolution of 600 dots per inch, 4992 dots of bit data are shifted sequentially through a shift resistor comprised of flip-flops FF1, FF2, . . . , FF4992. Then, the latch signal HD-LOAD is input into the LED head 78, so that each dot of bit data is latched in each of the latches LT1, LT2, . . . , LT4992. Of the light emitting elements LD1, LD2, . . . , LD4992, those assigned to dot data at a high level are lit in accordance with the bit data and a print drive signal HD-STB-N. In this figure, G0 denotes a NOR gate, G1, G2, . . . , G4992 denote NAND gates, TR1, TR2, . . . , TR4992 denote switching devices, and VDD denotes a power source.
Next, structures of the LED head and a focusing rod lens array are explained with reference FIG. 34. As shown in FIG. 34, the LED head 78 is constituted by LED chips 28 each having light emitting elements, a printed circuit board 27 on which driver IC chips for driving the LED chips 28 are arranged in a line, and a lens array 29 for condensing the lights emitted by the light emitting elements. The lens array, which is used for condensing the lights emitted by the light emitting elements and forming an image on the photosensitive drum, is comprised of a plurality of rod lenses spaced uniformly.
Of the parameters specifying optical characteristics of the lens array, one is the MTF (Modulation Transfer Function). The MTF is explained below with reference to FIG. 35. The MTF, which is one of the techniques for describing an optical system in terms of a frequency characteristic, describes how a spectrum (amplitude) of a spatial frequency at an input differs from that at an output. The MTF can be expressed in equation form shown below as a response function of SLA. The value of the MTF can be calculated on the basis of intensity of the light received by a CCD image sensor when an input image in a grid pattern as shown in FIG. 35 is input into the lens array and an output image is formed on the CCD image sensor disposed at the output side of the rod lens array.MTF(w)=(i(w)max−i(w)min)/(i(w)max+i(w)min)×100(%)
In the above equation, (w) max and i(w) min respectively represent a maximum and a minimum of the output image responsive to the input image in the grid pattern for a particular spatial frequency w (lp/mm). FIG. 36 shows an example of the values of the MTF obtained by measuring the light intensity at each dot position of the LED head (at each of the positions facing the light emitting elements respectively).
FIGS. 37(a) and 37(b) show light intensity distributions when three consecutive light emitting elements are lit. As shown in FIG. 37, a light intensity distribution of a light emitting element (LED) is analogous to the Gaussian distribution.
In FIG. 37(a), the curves a1, a2, a3 represent light intensity distributions of the lights emitted by the three light emitting elements, and the curve b represents a light intensity distribution of a combination of the lights emitted by the three light emitting elements. As shown in this figure, when the neighboring light emitting elements are lit, the lights emitted by them exert their effects mutually, thereby making the combined distribution as shown by the curve b. The same holds true for the curves a1, a2, a3 and the curve b in FIG. 37(b).
Even if the light emitting elements are configured to form dots (light spots) of a uniform diameter, when neighboring light emitting elements are lit at the same time, the diameters of the dots vary, since their MTF values may vary and therefore each light emitting element receives different influence from adjacent light emitting elements. FIG. 37(a) shows a case where the MTF value of the middle light emitting element is small. If the MTF value is small, it is difficult to form a sharp dot image and the diameters of the dots adjoining the middle dot are increased, since the light emitted by the middle light emitting element exerts large effect on the adjoining dots. FIG. 37(b) shows a case where the MTF value of the middle light emitting element is large. If the MTF value is large, the dot image becomes too sharp and the diameter of the middle dot is increased, since the lights emitted by the adjoining light emitting elements are incorporated into the light emitted by the middle light emitting element. In short, if the MTF value of any of the light emitting elements is different from a normal value, the combined light intensity distribution is distorted whether it is larger or smaller than the normal value.
In order to remove dot-to-dot variation in the diameters due to variations of optical characteristics of the light emitting elements, it is known to correct printing data to be supplied to the light emitting elements in accordance with correcting data so that all the light emitting elements emit the light at the same intensity. The correcting data is obtained by causing the light emitting elements to emit the light one by one and measuring the intensity of the light sequentially. With such correcting data, it is possible to make the diameter of any dot equal to a certain set value as long as each of the light emitting elements emits the light alone. However, there is a problem that the dot diameter variation cannot be resolved to a sufficient degree, since, when neighboring light emitting elements emit the light at the same time, some dots may have diameters that differ from the set value.
The reason is that each of the light intensity distributions of neighboring light emitting elements has effect on the combined light intensity distribution as explained with reference to FIG. 37, and the effect depends on optical characteristics of the rod lenses that vary and intervals between adjacent rod lenses that slightly vary, and therefore the diameters of the formed dots may vary even if the light emitting elements are configured to form light spots having a uniform diameter. As a result, an electrostatic latent image formed on the photosensitive drum is distorted and accordingly a toner image distorted, causing degradation in printing quality.